Many opto-electronics devices, such as direct modulated lasers, electro-absorption modulated lasers, and interferometric optical modulators, suffer from distortion due to nonlinear effects that are present in the device in certain signal intensity ranges. At low or moderate input signal intensities, one or two nonlinear terms, proportional to an even power term (e.g., 2 or 4) or proportional to an odd power term (e.g., 3 or 5), often dominate the nonlinear portion of a response. Ideally, it should be possible to compensate exactly for these lowest order nonlinear distortion terms by removing such terms, preferably as a pre-distortion signal.
Several classes of techniques for compensation for presence of nonlinear signal distortion have been developed. A feed-forward technique is capable of achieving suppression of distortion of around 18 dB, with individual controls for even-(second) and odd-(third) order suppression. A general non-linear transfer function can be synthesized in principle by filters/equalizers and a delay line. However, implementation and adjustment is often complicated.
Use of a parametric feedback method is possible only for devices that allow distortion control with an external dc voltage (e.g., second order control for a Mach-Zehnder interferometer).
An in-line technique is simple to implement, but makes some major compromises. One approach uses independent real and imaginary distorters, which are located in the shunt path of the signal flow and thus disturb signal matching. For this reason, the shunt loading by the distorter must be kept small so that only a small amount of controlled distortion can be generated. As a result, these linearizers work well when the device to be linearized does not have appreciable distortion components. Also, it is difficult to separate the real and imaginary parts of a signal completely, and the controls become inter-dependent. The isolation between real and imaginary distortion components is sometimes attempted by amplifiers (usually MMIC's), which are sources of distortion themselves. All these factors limit the performance. Furthermore, these approaches are part of a class of lossy linearizers with the loss approaching zero, and their power handling capability is severely compromised.
Another approach addresses linearization as a purely real part (in-phase) problem. However, even if the non-linear transfer function (NTF) to be compensated is real, the parasitics of the linearizer elements would limit the performance at higher frequencies. Therefore, unless reactive compensation is used, this type of circuit would not be capable of operating over a wide bandwidth. If an imaginary part of the NTF is to be generated, this reference does not indicate how to achieve this. Therefore, this approach would be limited to systems of limited bandwidth and for linearization of real NTFs. This approach also does not teach how to synthesize a second-order (or, more generally, an even-order) non-linearity by itself or in conjunction with some odd-order non-linearity.
What is needed is an approach that provides broadband linearization and reduces odd order and even order signal distortion. Preferably, the approach should provide compensation for separate even order and odd order nonlinear distortion, for combined even and odd order distortion, and for expansive and compressive distortion. Preferably, the approach should provide one or more controllable parameter values that can be used to match the coefficients associated with anticipated nonlinear distortions.